In the field of Automatic Optical Inspection (AOI), depth and distance measurements occupy an important position. Some technologies, such as chromatic confocal spectroscopy, white light interferometry, conoscopic holography, triangulation, depth from focus (DFF) and depth from defocus (DFD) can perform such measurements.
Depth from defocus consists in obtaining depth information from the analysis of the amount of defocus within one or more captured images. Active depth from defocus can be seen as an extension of depth from defocus, and consists in focusing a light beam into a spot onto the surface of the object to be inspected and deduce depth information from the size of the spot of light. The depth precision and measuring range depends on the type of light source (LED, laser, etc.) and the characteristics of the optics used to collimate and focus the light beam. Once the focusing lens is chosen, the measuring range and precision are fixed. Such approach is inadequate for inspecting parts requiring different measuring range and depth precision.
In depth from focus, in-focus Regions Of Interest (ROI) are extracted from images of an object taken at different focus distances in order to obtain a stack of 2D slices, these slices are then used to reconstruct a 3D model of the object. Here, the resolution and accuracy of the measurement system depend on the depth of field of the optical system.
Active depth from defocus is adequate for material presenting proper reflectivity and little diffusion (scattering) while depth from focus requires the surface of the object to be inspected to present sufficient texture so that in-focused ROI can be reliably extracted. One system doesn't allow for inspecting both types of material. There is therefore a need for a depth probe incorporating a mean to change the measuring range, and for a depth probe allowing characterizing textured surfaces as well as surfaces offering a specular reflection.